With the development of communication and the spread of multimedia technology, a wireless communication system uses various techniques for massive data transmission. A method of assigning a more number of frequency resources may be used as a method of increasing radio capacity. However, since the number of frequency resources is limited, there is a limitation when the more number of frequency resources are assigned to a plurality of users. As one of methods for effectively utilizing the limited frequency resources, there is a method of creating a small-sized cell. When the small-sized cell is created, one base station (BS) provides a service to a less number of users, and thus the BS can assign a more number of frequency resources to the users. By creating the small-sized cell, a service for massive data transmission with better quality can be provided to the plurality of users.
To increase system efficiency and to improve quality of service (QoS) in an indoor environment, a protocol standardization work for a wireless access system supporting a femto-cell is in progress in the institute of electrical and electronics engineers (IEEE) 802.16 task group which defines a next generation wireless interface standard and in the worldwide interoperability for microwave access (WiMAX) forum which is an non-profit organization providing a service and a network protocol for an IEEE 802.16-based broadband wireless access system. In the WiMAX forum, a femto BS is defined as a low-power low-cost BS connected with an Internet protocol (IP) network through a fixed wireless link or a local broadband wired link. The femto BS is connected to the IP network widely used in homes or offices, and provides a mobile communication service by accessing a core network of a mobile communication system. That is, the femto BS may be connected to the core network of the mobile communication system through a digital subscriber line (DSL). A user of the mobile communication system may receive a service via a conventional macro-cell in an outdoor environment, and may receive a service via a femto-cell in an indoor environment. The femto-cell improves indoor coverage of the mobile communication system by correcting a problem in which the conventional macro-cell provides a deteriorating service inside a building. Since the femto-cell can provide a service only to a specific designated user, a voice service and a data service can be provided with high quality. Further, the femto-cell can provide a new service not provided by the macro-cell. With the wide use of the femto-cell, fixed-mobile convergence (FMC) can gain a momentum, and industry-based costs can be reduced.
In general, a cell coverage of the femto BS is smaller than a cell coverage of the macro BS, and the cell coverage of the femto BS may be entirely or partially included in the cell coverage of the macro BS. In a case where a user equipment (UE) belonging to the cell coverage of the macro BS performs communication, if the UE also belongs to the cell coverage of the femto BS, it may be advantageous to perform a handover from the macro BS to the femto BS. The same also applies in the opposite case. A handover may be initiated by the UE, the macro BS, or the femto BS. A handover initiated by the UE is a UE-initiated handover. A handover initiated by the macro BS is a BS-initiated handover.
The UE-initiated handover may be performed by the UE by directly measuring a signal of a neighbor BS and transmitting to the macro BS a message for requesting a handover to a desired BS. On the other hand, the BS-initiated handover may be performed by the macro BS by transmitting a handover command message to the UE.
The BS-initiated handover may also be performed by the femto BS. In this case, the femto BS directly scans neighbor UEs and allows a desired UE to initiate a handover from the macro BS to the femto BS. However, a method of performing a handover initiated by the femto BS has not been clearly defined up to now. Accordingly, there is a need for an apparatus and method for performing a handover in a wireless communication system employing a macro BS and a femto BS.